De Sincléir
de SINCLÉIR—XI—de Seynclere, de Sencler, Sinclare, Sinclair, St. Clair, St. Clare, etc.; Norman 'de St. Clair,' Latin 'de Sancto Claro,' 'de Sancto Clero,' i.e., of St. Clair, a frequent place-name in Normandy. This surname came into England with William the Conqueror, and was borne by an important Scottish clan. In Ireland also it occurs early, but the family was never either numerous or important.
Alphabetical Index to Irish Surnames